JPH0687193A - Multilayer stretch/shrink film - Google Patents

Abstract

PURPOSE: To obtain a heat-shrinkable film useful for a food over-wrap wherein poultry or the like is put in a tray by biaxially stretching a film consisting of outer layers comprising a mixture of a specific ethylene/α-olefin plastomer copolymer and VLDPE and a core layer comprising a specific ethylene/α-olefin copolymer. CONSTITUTION: A heat-shrinkable film comprises at least three layers and outer layers comprise a mixture of 20-35 wt.% of ethylene/α-olefin plastomer copolymer with a density of 0.90 g/cm<3> or less and 65-85 wt.% of VLDPE with a density of 0.914 g/cm<3> or less and a core layer comprises an ethylene/α-olefin copolymer having an m.p. higher than that of either one of both outer layers. The content of the plastomer copolymer in the outer layer mixture is pref. 22-30 wt.% and that of VLDPE therein is pref. 70-78 wt.%. This three-layered film functions in a degree of a conventional five-layered film and the power of required in exhaustion airtight sealing is about half and a large shrinkage factor and small permanent strain % are brought about.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to an improved biaxially stretched heat-shrinkable multilayer film and a poultry-type, hermetically sealed tray in which the film is in a heat-shrinkable stretched state as an overlap. Food packages that have been and have been evacuated.

[0002]

BACKGROUND OF THE INVENTION Food products, such as fresh poultry, have been shipped in bulk from food processors to supermarkets where bulk pieces have been divided into smaller pieces and repackaged for retailing for many years. For example, fresh poultry was cut, placed on cardboard or plastic trays, wrapped in stretched film, and stretched film was tray fixed by tack welding. In order to improve efficiency, the current trend is to carry out retail packaging operations in food processing plants from which small retail packages are shipped to retailers. The longer storage time between retail packaging and consumer use also tends to evacuate and hermetically seal food retail packages in central food processing plants. Such packaging prolongs the shelf life of food packages. There is also a need for food retail packages with increased abuse resistance due to the increased frequency of handling, impact and wear inherent in the food processing plant retail packaging systems described above.

The most commonly used film in the tray food overlap market is polyvinyl chloride (PV
C). This thermoplastic polymer has been used because of its satisfactory elongation and elastic memory. Elastic PVC shrinks as the food in the tray loses water and the food itself shrinks. As a result, the package can be tight and leaky, which can in some cases be unobtrusive. This is because the PVC material in the overlapping trays is tack welded to itself and is not hermetically sealed. PVC
Leakage may occur during tack welding as the contracts. As a result, liquid that oozes from overlapping food items (eg, meat) may come out of the package. This problem is due to the effective seal (at the low temperature end) and PVC "burn through". ) ”Or melting (at the higher temperature end), the sealing temperature range is too narrow to be solved by a hermetic seal. This range is so limited that it is not feasible for many heat sealing systems used commercially.

Another limitation of PVC tray overlap materials is their poor physical abuse resistance. The PVC material tends to tear along the edges of the overlapping trays when rubbed by another tray or an enclosing carton during shipping.

One of the properties of PVC is that it shrinks but is not "heat shrinkable", that is, it tends to return to its original unstretched (unstretched) dimension when heated to its softening point. It is not a material in. The term "stretched" or "stretched" is used to refer to the production of heat-shrinkable films that heat a resin material to a flow or melting point and extrude through a die into a tubular or sheet form. . After cooling, the relatively thick extrudate is reheated to a temperature range suitable for orienting or aligning the crystallites and / or molecules of the material. Those of ordinary skill in the art will understand that the stretching temperature range for a given material or materials may be altered by changing the intermolecular structure by physically aligning the crystallites and / or molecules of the material, for example according to ASTM D-2838-81. It is understood that there is a range that improves certain mechanical properties of the film, such as shrink tensile stress as measured. Uniaxial stretching occurs when the stretching force is applied in one direction. If the stretching forces are applied simultaneously in two directions, biaxial stretching will occur.

In view of the limitations of PVC as a stretched film food tray overlap material as described above, an attempt is made to identify the identity of heat shrinkable thermoplastic films with improved combinations of elongation, elastic memory, heat seal suitability and fracture resistance. There has been an effort to do so. However, most heat-shrinkable thermoplastic film packaging materials suitable for contact with foods
Elasticity or elastic memory is relatively poor. Thus, when food wrapped in such materials shrinks due to water loss, the film does not shrink resulting in an aesthetically unpleasant package.

One of the heat-shrinkable biaxially stretched films that has been used commercially in the trayed food overlap market is W. R. Grace and Company-Connecticut is a five layer film labeled SSD-310. Although this polyolefin-containing composition has many of the above-mentioned properties, when it is used to seal the food storage tray after it has been evacuated, the power consumption of the impulse and heat sealing devices is relatively high. This is due in part to the fact that SSD-310, due to its special structure, is radiation crosslinked in order to achieve bubble stability during biaxial stretching and also abuse resistance. This is because it must be done. Another reason for higher power consumption is that SSD-310 film is 350 ° to 430 ° F (177 ° to 221 ° C).
A high sealing temperature is required, which in turn requires a long residence time.

Another important film property for food tray overlap applications is low permanent set. Permanent set is a measure of the increase in length of a film after it has been stretched and relaxed. More specifically, the permanent set test used in the present invention is from ASTM 621 and measures the percent increase in length of a film sample stretched 50% and then relaxed for 30 seconds. A lower percentage value reflects that the film can relax after being stretched. In tray-overlap applications, good film recovery retains package appearance and integrity after handling abuse. The perfect elastomer has 0% permanent set. Plasticized PVC typically has a strain of 3-5%, and polyolefins such as LLDPE typically have significantly higher permanent strain, on the order of 6-15%.

One of the objects of the present invention is to provide an improved heat-shrinkable stretched film suitable for use as a trayed food overlap material. Another object of the present invention is to provide such a film that is capable of forming a food package contained in a hermetically sealed tray as a stretch overwrap material.
Yet another object is to provide such a film having high abuse resistance. A further object is to provide such a heat-shrinkable stretched film in the form of 5 or less multilayers. A further object is to provide a stretched film that can be biaxially stretched without irradiation crosslinking.

An additional object is to provide such a film as a stretch wrap material around food storage trays which can be hermetically sealed at lower temperatures and with less energy than previously required. Another object is to provide a heat-shrinkable stretched multilayer stretched film containing a polyolefin having a small permanent set. A still further object of the present invention is to provide an improved evacuated, hermetically sealed food containing tray package that is overlapped by a heat shrink film. Another object is to provide a highly abuse-resistant tray food package that is evacuated by a heat-shrinkable plastic film overlap and is hermetically sealed. Other objects and advantages of the invention will be apparent from the disclosure and claims that follow.

[0011]

One aspect of the present invention is a biaxially stretched heat-shrinkable multi-layer stretch comprising at least a first outer layer, a second outer layer and a core layer between the first and second outer layers. Regarding film. The first outer layer and the second outer layer each comprise about 20 to about 35 wt% ethylene alpha-olefin plastomer copolymer having a density less than about 0.90 g / cm ^3.
And a VLDP with a density less than about 0.914 g / cm ³ .
E about 65 to about 80% by weight of the blend. From this specification, the term "ethylene alpha-olefin plastomer" is further described. The core layer comprises an ethylene alpha-olefin copolymer having a melting point above the melting point of either the first outer layer or the second outer layer. "Heat-shrinkable" as used herein means that the film is ASTM D
It is meant to have at least 20% free shrinkage at 90 ° C. measured in at least one direction according to −2732, ie longitudinal or transverse.

Another aspect of the invention is a bottom section, the bottom section being surrounded by upwardly extending sidewalls, a perishable food product supported on the top surface of the bottom section,
And the food, along with each of the upper edges of the side walls and at least a portion of the bottom surface of the tray bottom section, and heat sealed to itself in a flattened relationship to the bottom surface, so A hermetically sealed and evacuated food package comprising a stretched heat shrink film adapted to form a hermetically sealed enclosure. In this aspect of the invention, the feature is a biaxially stretched multilayer composition as a stretched heat-shrink film that includes at least a first outer layer, a second outer layer, and a core layer between the first and second outer layers. is there. Each of the first outer layer and second outer layer has a density of about 0.90 g / cm ³ less than the ethylene alpha - olefin plastomeric polymers about 20 to about 35 wt% and a density of about 0.914 g / cm ³
Smaller VLDPE including about 65 to about 80% by weight blend. The core comprises an ethylene alpha-olefin copolymer having a melting point above the melting point of either the first outer layer or the second outer layer.

DETAILED DESCRIPTION OF THE INVENTION The multilayer stretched film of the present invention requires at least three layers, a first outer layer, a second outer layer and a core layer between the first and second outer layers. The enclosure must be evacuated and hermetically sealed to enclose and store perishable products such as foods with this film. Sealing is by heat, that is, heating the film enclosure to a range between the heat seal initiation temperature and the burn-through temperature. More specifically, in order to obtain effective heat sealing, the film is heated to a temperature close to the melting point of the first or second outer layer forming at least the inner layer of the package surrounding the perishable product. Since these layers contain a mixture of two types of ethylene alpha-olefins that approximates a true homogeneous blend, the melting point of the blend is between those of the two major (ethylene alpha-olefin) components, Approach temperature based on a linear relationship that reflects the relative proportions.

When the melting point of the core layer is equal to or lower than the melting point of the inner layer surrounding the package, the whole multilayer film is melted, so that the strength is almost lost, and the heating means penetrates into all three layers to form a film. Will be cut off, that is, burn-through will be performed. This problem is avoided by employing a core layer having a melting point higher than that of the first and second outer layers. Thus, when heating the inner layer to its melting point for heat sealing,
The core layer does not melt and the multilayer film retains its strength and integrity as a heat sealed but shrunk enclosure.

Very low density polyethylene (hereinafter referred to as V
LDPE is also called "ultra low density polyethylene (ULDP
Also referred to as E), copolymers of ethylene with alpha-olefins, usually 1-butene, 1-hexene or 1-octene, in some cases e.g. terpolymers of ethylene with 1-butene and 1-octene. Including terpolymer. VLDPE as used in the present invention also includes terpolymers of ethylene and higher alpha-olefin comonomers. VLDPE has a lower density than linear low density polyethylene (LLDPE),
It is a different class of material than LLDPE and will be recognized by those skilled in the art as ranging from about 0.86 to about 0.914 g / cm ³ . A method for manufacturing VLDPE is described in European Patent Publication No. 120,503, the text and drawings of which are incorporated herein by reference.

For example, as described in US Pat. Nos. 4,640,856 and 4,863,769,
VLDPE is used in biaxially stretched film, LLD
It can have superior properties to films comparable to PE. These excellent properties include high shrinkage, high tensile strength and high fracture resistance. VLDPE is commercially available as either a powder or granules in dry resin form and is manufactured and sold by, for example, Union Carbide Corporation and Dow Chemical Corporation.

Ethylene alpha-olefin copolymers are also made to be elastic and are referred to hereafter as "ethylene alpha-olefin plastomers". Some of these have densities in the same range as VLDPE, but have significantly different physical properties due to different manufacturing processes. For example, ethylene alpha-olefin plastomers and VLDPE are made using different catalyst systems. One of the manufacturers of these materials is Mitsui Petrochemical Co., Ltd., which manufactures ethylene-butene plastomer copolymers under the name "Tafme".
It is sold under the name "r". The company uses ethylene alpha
It is interesting to note that the olefin plastomers and also the company manufactures and sells both LLDPE and the materials it characterizes, the two types of materials being manufactured in different plants with different catalyst systems. Is.

The company has two types of ethylene alpha-
Differences in olefin copolymers from October 1991 2
4th "Proceedings of Future-
Pak '91, Ninth International
al Ryder Conference on Fo
od Packing Innovations "
It has been described in. According to US Pat. No. 4,469,753, Tafmer is a copolymer of ethylene and butene-1. Ethylene alpha as used in the present invention
Olefin plastomers include terpolymers of ethylene and higher alpha-olefin comonomers. As discussed in detail hereinbelow, VLDPE
And ethylene alpha-olefin plastomers can be compared by crystallinity, Vicat softening point, molecular weight / size distribution, and tensile properties. For the purposes of the present invention, they are broadly distinguished on the basis of at least the properties of the resin forms (Table 1) below:

[0019]

[Table 1]

Depending on the melting points of the first outer layer and the second outer layer, the core layer having a higher melting point is, for example, VLDPE or LLDP.
It may be E, polypropylene, ethylene-propylene copolymer, butene-1, or a blend of at least two ethylene alpha-olefin copolymers such as low density and high density materials.

As will be described in more detail hereinafter, the three-layer film overcomes the above-mentioned limitations of biaxially stretched heat-shrinkable films as PVC substitutes for tray food overlaps. However, it works as well as the five-layer film used in commerce and requires about half the power required to seal a food package in an evacuated tray, which is even greater under uniform sealing conditions. Provides seal strength. Moreover, this three-layer film has proven to provide an unexpected combination of high shrinkage values and low permanent set percentages.
These properties are all advantages in food packaging and applications, especially for the food processing plant retail packaging system described above which stretch-wraps trayed food with plastic film.

The preferred ethylene alpha-olefin plastomers in the practice of the present invention are T manufactured by Mitsui Petrochemical Co., Ltd. as a butene-ethylene copolymer.
afmer A-4085. Other suitable ethylene alpha-olefin plastomers are also Tafmer types A-4090, A-1085, P from Mitsui.
-0480 and P-0680. According to the manufacturer, P-type Tafmer is amorphous, whereas
A-type Tafmer has a low crystallinity and, due to this property, has a slightly higher tensile property. These materials are based on "Tafmer A / P" published by Mitsui and "Pr" mentioned above.
ocedings of Future-Pak 9
1 ”publication. According to the manufacturer, these ethylene alpha-olefin plastomers have the physical properties of the resin forms summarized in Tables 2 and 3 below.

[0023]

[Table 2]

[Table 3]

The above-mentioned Tafmer A-4085 is
It is suitable for end use applications where the optical properties of the inventive film are important, such as the stretched overlapping tray food product described above. The criterion for choosing this is that it has a high gloss and a low haze under comparable conditions, which is due to VLDPE (density 0.906 g / cm ³ , melt index 0.8) 85% and ethylene alpha-olefin plastomer. Proven in a series of tests to make blown (unstretched, non-heat-shrinkable) films from several formulations containing 15% copolymer. The latter is three Tafm
er material, and the properties were compared to those of 100% VLDPE. In these tests, the extrusion temperature was 3
34 ° to 341 ° F (168 ° to 172 ° C) and the die size is 21/2 inch x 0.0315 inch (6.35).
cm × 0.0800 cm) and the resulting blown film tube had a fold diameter of 10.125-10.5 inches (25.718-26.7 cm). The results are summarized in Table 4 below.

[0025]

[Table 4]

Table B shows that a film containing 15% Tafmer A-4085 is comparable to Tafmer.
It shows that the haze percentage is much lower and the gloss percentage is much higher than the P-0480 and Tafmer P-0680 containing films, or the 100% VLDPE films. From these tests, from a qualitative standpoint, even if they were blown films,
The optical properties of the biaxially stretched heat-shrinkable film of the present invention using Tafmer A-4085 are:
It can be concluded that it is superior to the optical properties of comparable films with P-0480 or P-0680. VLDPE suitable for practicing the present invention are manufactured by Dow Chemical Company and Union Carbide Corporation and include those having the physical properties of the resin form as summarized in Tables 5 and 6 below according to the manufacturer.

[0027]

[Table 5]

[Table 6]

Permanent Strain The procedure used to measure permanent set in the examples below is AST
Derived from MD621, using an Instron table model tensile tester or equipment. MD four samples
The (longitudinal) permanent set is tested and each is cut to an MD length of 5 inches (13 cm) and a TD (transverse direction) length of 1 inch (2.5 cm). Similarly, four samples are tested for TD set and each is cut to a TD length of 5 inches (13 cm) and an MD length of 1 inch (2.5 cm). Set the chart speed for both A and B jaws to 20 inches / minute (51 cm / minute). Set the A jaw crosshead speed to 2 inches / minute (5 cm / minute) and the B jaw crosshead speed to 20 inches / minute. Adjust the chart pen settings so that the crosshead returns to a gauge length of 2 inches after stretching the film 50% of its original length, or 1 inch. B
Performs a jaw speed of 20 inches / minute with a crosshead of 5
After stretching by 0%, it returns to its original position. This original position is 30
Hold for a second. A jaw speed of 2 inches / minute is similarly performed. When the force begins to reappear on the chart, bring the crosshead back to its original gauge length. Read the chart in inches from the beginning of the test to the reappearance of force, ie where the pen leaves the baseline. Multiplying the inches on the chart by 5 gives the percent permanent set. Repeat the procedure for the remaining samples and calculate the arithmetic mean. The procedure is otherwise the same as ASTM 621.

Shrinkage The biaxially stretched stretched film of the present invention is "heat shrinkable" which, as used herein, means that the film is at 90 ° C in both the machine and transverse directions. It is meant to have an unrestrained shrinkage of at least 20%. The shrinkage value is obtained by measuring unconstrained shrinkage of the stretched film at 90 ° C. for 5 seconds. Four specimens are cut from a given sample of stretched film to be tested. The test piece is cut into 10 cm in the longitudinal direction and 10 cm in the transverse direction. Each test piece is completely immersed in a 90 ° C. water bath for 5 seconds.
Measure the distance between the ends of the contracted specimen. The difference between the measured distance and the original 10 cm for the shrunken specimen is multiplied by 10 to obtain the percent shrinkage for the specimen. Shrinkage for four specimens is averaged over MD shrinkage values for a given film sample, and shrinkage for four specimens is averaged over TD shrinkage values.

Shrinkage Force The shrinkage force of the film is the force or stress required to prevent the shrinkage of the film, and was determined from the film samples taken from each film. Four film samples were cut 1 inch wide (2.54 cm) by 7 inches long (17.8 cm) in the machine direction and 1 inch wide by 7 inches long in the cross direction. The average thickness of the film sample was determined and recorded. Each film sample was then clamped between two clamps 10 cm apart. One clamp is in a fixed position and the other is connected to a strain gauge transducer. The fixed film sample was then immersed for 5 seconds in a silicone oil bath maintained at a constant elevated temperature. During this time, the force in grams at elevated temperature was recorded. At the end of this time, a film sample was removed from the bath and allowed to cool to room temperature, and the force, in grams at room temperature and at room temperature, was recorded. Then, the shrinkage force for the film sample was determined from the following formula. Results are obtained in grams per mil of film thickness (g / mil): Shrink force (g / mil) = F / T where F is the force in grams and T is the film sample in mils. Is the average thickness of.

The crystallinity PLAST mer type ethylene alpha - both manufacturers olefin copolymers and VLDPE is, the crystallinity of usually plastomers have reported that small compared to VLDPE copolymers. In particular, Mitsui Petrochemical Co., Ltd. reports that its Tafmer has a lower degree of crystallinity than its VLDPE material (October 2-4, 1991, “Future-Pak '91, Nint.
h International Ryder Con
ference on FoodPacking
Innovations, p. 314). Mitsui
For comparable densities, the Tafmer-type ethylene alpha-olefin copolymers of the same company have a crystallinity of 10-15%, while the VLDPE of the same company
Indicates that it has a crystallinity of 25-35%.

Differential scanning calorimetry (DSC) is commonly used to measure the amount of crystallinity in plastic samples and also reveals the nature of this crystallinity.
For example, DSC is performed by exposing a sample of plastic to a constant heating rate, ie 50 ° C./min and a DuPont 9000 brand differential scanning calorimeter, as determined by a procedure similar to ASTM D-3418, for example. When the temperature of the sample reaches the melting point of the crystalline region, continued application of heat causes the crystalline fraction to melt, the sample temperature remaining constant during this process. After the crystalline region has melted, the sample temperature begins to rise again. DSC measurements were made on two types of VLDPE: Union Carbide 1137 (ethylene-butene copolymer) with density 0.906 and Dow Attane with density 0.912.
(Ethylene-octene copolymer). The same type of measurement was performed on Tafmer A-4085. While each of these ethylene alpha-olefin copolymers has a degree of crystallinity, the crystalline properties of ethylene alpha-olefin plastomers and VLDPE are quite different.

Ethylene alpha-Tafmer A-4
The entire crystalline phase of 085 plastomer melts at about 55 ° -85 ° C, a melting point range consistent with crystalline phases made of ordered butene-ethylene copolymers. In contrast, VLDPE copolymers have at least two crystalline phases, the main phase being the high melting point phase, in each case about 117 ° to 12 °.
It is in the range of 5 ° C. This is representative of what ethylene homopolymers exhibit, and in essence the plastic consists of two copolymers: a higher ethylene alpha-olefin-.
It could be considered as ethylene copolymers and composites of ethylene copolymers. The melting points of representative VLDPE and ethylene alpha-olefin copolymer plastomers are summarized in Table 7 below.

[0034]

[Table 7]

Table D demonstrates the large difference in melting points between VLDPE and plastomers. In more detail,
The melting points of ethylene alpha-olefin copolymer plastomers are below about 90 ° C and the melting points of VLDPE materials are above about 90 ° C. The melting point of the plastomer is preferably below about 85 ° C. Based on the above, suitable VLDPE copolymers have a crystalline melting point of about 100 ° to less than about 125 ° C. Resins with a crystalline melting point above about 125 ° C become increasingly difficult to process as biaxially stretched heat-shrinkable stretch films for food packaging as the temperature increases. VLDPE resins useful in the outer layer blends of the invention preferably have a crystalline melting point of about 115 ° to about 125 ° C.

[0036] As previously described Vicat softening point, VLDPE materials ethylene alpha plastomer type - in and penetration difficult tendency lower flexibility than the olefin. Plastomer type ethylene alpha-olefins are more flexible and soft. This is shown by comparing the Vicat softening points for the two copolymers. Vicat softening point is A
Circular cross section 1 mm ² as specified in STM1525
Is the temperature at which the flat tip needle penetrates the thermoplastic test piece to a depth of 1 mm using a uniform heating rate selected under a specific load. Vicat softening points for various VLDPE and plastomer-type ethylene alpha-olefins have been reported by resin manufacturers and are summarized in Table 8 below.

[0037]

[Table 8]

It should be noted that Union Carbide 1085 material has a much lower Vicat softening point than other VLDPEs, but is still considered a VLDPE due to other properties. Based on the above and for the purpose of the present invention, VL
The DPE-type ethylene alpha-olefin copolymers preferably have a Vicat softening point of at least about 68 ° C, most preferably about 78 ° to about 100 ° C. Conversely, plastomer-type ethylene alpha-olefin copolymers preferably have a Vicat softening point below about 68 ° C, and most preferably between about 50 ° and about 65 ° C.

Molecular Weight / Size Distribution Ethylene alpha-olefin copolymers can be characterized in part by the weight average molecular weight (M _w ). Weight average molecular weight is determined by multiplying the weight of each chain of a given number of repeating units by the number of such chains and dividing by the total weight of the chains. Ethylene alpha-olefin copolymers can also be characterized by a partial number average molecular weight ( _Mn ). The number average molecular weight is obtained by dividing the total weight of polymer molecules by the total number. Knowing both of these, they can be used to characterize the shape of the molecular weight distribution curve for the copolymer, ie the number of polymer chains in the molecular weight interval as the ordinate and the molecular weight as the abscissa. Qualitatively speaking, M _w / M _n
Is large means that the molecular weight distribution is wide, whereas small M _w / M _n means that the distribution is narrow. M _w / M _n can be measured by a number of different techniques, but as used in the present invention, ASTM D
The gel permeation chromatography ("GPC") procedure outlined in -3593-80 is used.

The relatively narrow M _w / M _n distribution (and the small ratio) allows for a sharper control of physical properties and excellent optical properties, ie relatively high gloss and small haze. Is normal. However, polymers with relatively low M _w / M _n ratio values are also difficult to process. More specifically, certain ethylene alpha-olefin plastomers known to the Applicant to be useful in the present invention have a relatively narrow molecular weight distribution and _Mw.
/ M _n value is smaller than about 3. They are relatively difficult to process into films by melt extrusion. Although they can be melt extruded into blown films, Applicants have noted that Tafm
Biaxially oriented heat-shrinkable films could not be made from these materials such as er A-4085.

All of the specific VLDPE copolymers known to Applicant and available in commercial quantities by the filing date of the present application are known to Applicants to be useful in the practice of this invention. Significantly wider molecular weight distribution and larger M _w / M than ethylene alpha-olefin plastomer
_{has n} values. Therefore, VLDPE copolymers have greater impact strength, tensile strength, wear resistance and better processing properties. For example, these VLDPEs may themselves be melt extruded into primary tubes and then biaxially stretched into heat shrinkable tubes. The molecular weight / size distribution ratios for representative commercially available ethylene alpha-olefin copolymers are summarized in Table 9 below.

[0042]

[Table 9]

These values are AST unless otherwise indicated.
It was measured by the GPC procedure of MD-3593-80.
Plastomer type M _w / M _n based on commercially available ethylene alpha-olefins reported in Table F.
It is clear that the _Mw / _Mn value of the VLDPE copolymer is greater than about 3, while the value is less than about 3. More specifically, known plastomer-type ethylene alpha-olefins have _Mw / _Mn values of less than about 2.5, while commercially available VLDPE materials preferably have _Mw / _Mn values. Having at least about 5. But,
It may be possible to produce VLDPE materials with a relatively narrow molecular weight distribution and M _w / M _n values of less than about 3.

VLDPE of known tensile strength typically has a greater tensile strength than comparable ethylene alpha-olefin plastomers. That is, VLDPE has greater crystallinity and density if two copolymers are made from the same comonomer and using the same catalyst system,
Therefore, it has a higher tensile strength. Mitsui, a manufacturer of both plastomers and VLDPE, reports the following values for Young's Modulus (the ratio of stress to strain below the material's proportional limit), as summarized in Table 10 below. is doing.

[0045]

[Table 10]

Another difference in tensile strength between the ethylene alpha-olefin plastomers and the VLDPPE material is that most VLDPs, whereas the former has no apparent yield point.
E material has such a point. The yield point is
Stress-as specified in ASTM D-638
In the strain curve, strain increase is the first point that occurs without stress increase. ASTM D-638 also defines yield strength as the stress at which a material exhibits a certain marginal deviation from the stress-strain proportionality relationship, which stress, unless otherwise specified, is the stress at the yield point. Become. In the above-mentioned Mitsui publication "Future-Pak '91", p. 314,
The following information has been reported on yield strength: Taf
No value reported for mer A-4090, VLDPE with a density of 0.896 g / cm ³ is 42 kg.
/ Cm ² and a density of 0.907 g / cm ³ of VLDP
E is 84 kg / cm ² . This shows that, in view of the manufacturers of both ethylene alpha-olefin plastomers and VLDPE, the former has no yield point and instead breaks when stressed well. In contrast, Mitsui shows that its VLDPE material has a well-defined yield point.

Mitsui's results show that various resin samples were
It was qualitatively confirmed in a series of tests made according to the procedure for thin plastic sheeting outlined in STM 882-90. The dimensions of these resin samples were as follows: width 1 inch (2.5 cm), length 4 inches (10 cm) and thickness 7-9 mils (0.18-).
0.23 mm). The yield point and tensile strength of these samples were determined according to Method A, initially at a constant grip separation rate, which held the ends of the specimen 2 inches (5.1 cm) apart (this rate was 20 inches / minute (51 cm / Minutes) was used. Five samples of each material were tested and the results averaged. The results of these tests are summarized in Table 11 below.

[0048]

[Table 11]

Union Carbide Type 1085VLD
It will be noted that PE, unlike other VLDPEs, did not have a yield point, like the ethylene alpha-olefin copolymer plastomers. However, it has other properties, including a melting point of 117 ° C.
Classified as E.

The biaxially stretched heat-shrinkable film of the present invention may be prepared by known techniques, eg, Canadian Patent No. 982923.
Can be prepared by co-extruding at least the core layer and the first and second outer layers on each side of the core layer as described in US Pat. Alternatively, after extruding the first outer tubular layer, by a coating laminating method for sequentially coating the outer surface of the first outer tubular layer and the core layer with the core and the second outer tubular layer, A primary tube may be formed. Alternatively, the first outer layer and the core outer layer may be coextruded by themselves and then the second outer layer may be coated on the outer surface of the core layer. The coating lamination procedure is described in US Pat. No. 3,741,
No. 253. As a further alternative, at least three layers of film may be formed into a sheet by the well known slot casting procedure.

If at least three layers of film are made as a primary tube or converted from a primary sheet to a tube, it can be biaxially stretched by the well known two step "double bubble" or trapped process. Good. One such process is Pahlke, US Pat.
56,044. This is accomplished by stretching the tube in the machine direction ("MD") by reheating the primary tube and simultaneously operating longitudinally spaced nip rolls at different speeds, as well as introducing air inside the tube. Transverse direction of tube ("T
D "). A suitable stretch ratio is about 2
It is about 6, and the MD / TD ratio is preferably about 3 to about 5.

Although not essential in the practice of the present invention, it may be desirable to crosslink one or more layers of the inventive film to improve abuse and / or puncture resistance and other physical properties. Absent. This may be done, for example, by irradiation with high energy electrons, ultraviolet light, x-rays, beta particles, etc. The irradiation source is about 150
It can be any electron beam generator that operates in the kilovolt to about 6 megavolt range and has an output capable of delivering the desired dose. The voltage can be adjusted to a suitable level, for example 1,000,000 or 2,000,000 or 3,000,000 or 6,0
It may be 0,000 or more or less. Many devices for illuminating the film are known to those skilled in the art. Irradiation is typically performed at doses up to about 20 MR, typically about 1 to about 20 MR, with a preferred dose range of about 2 to about 12 MR. Irradiation can conveniently be carried out at room temperature, but higher or lower temperatures may be used, eg 0 ° -60 ° C.

In connection with the process, irradiation can be applied to a single substrate layer, such as the first outer layer, and prior to biaxial stretching when the primary multilayer film is made by the coating lamination method. This type of irradiative crosslinking is described, for example, in the aforementioned US Pat. No. 3,741,253. Alternatively, when co-extruding the entire film at the same time, it may be preferable to irradiate the entire multilayer film, which
To maximize efficiency, see for example US Pat.
The dose level should be less than about 8 MR after biaxial stretching as described in US Pat. No. 4,638.

Crosslinking may also be accomplished chemically with peroxides, as is well known to those skilled in the art. John Wiley & Son
s, Inc. Published by and copyrighted in 1966 E
ncyclopedia of Polymer Sc
ence and Technology, Plas
tics, Resins, Rubbers, Fiber
s, vol. 4, pages 331-414. This document is Library of Congress C
atalog Card Number of 64-
22188.

The resins used in the present invention are usually commercially available in pellet form unless otherwise indicated, and commercially available equipment, including tumblers, mixers or blenders, as is generally recognized in the art. They may be melt blended or mechanically mixed by well known methods of use. Also, if desired, well known additives such as processing aids, slip agents, antiblocking agents, pigments, etc. and mixtures thereof may be incorporated into the film by blending prior to extrusion. The resin and all additives are introduced into an extruder (usually one extruder per layer) where the resin is melt plasticized by heating and then transferred to an extrusion (or coextrusion) die to form a tube. . Extrusion equipment and die temperatures are usually dependent on the particular resin or resin-containing mixture being processed, suitable temperature ranges for commercially available resins are generally known in the art, or resin production. See the technical bulletin made available by the vendor. Processing temperatures may vary depending on other process parameters selected. The ethylene alpha-olefin plastomer VLDPE of the present invention
When extruding the outer layer blend, the barrel and die temperatures are
For example, about 305 ° to about 350 ° F (152 ° to 177)
℃) range. However, changes are expected depending on such factors as other resins that may be used, the manufacturing process used and the particular equipment used and other process parameters. Those skilled in the art will set the actual process parameters, including process temperature, without undue experimentation.

A preferred embodiment of the heat-shrinkable film of the invention advantageously has a maximum extractable content in n-hexane of 50 ° C. for 2 hours of 5.5 weight percent of the polymer. This will be explained further below. This 5.5 weight percent is the maximum desired n- for the type of olefin copolymer employed by the present invention for use in food contact products except those used for packaging or holding while cooking food. Represents the hexane extractable limit. The maximum extractables as described above are advantageously 2.6 percent in a particularly preferred embodiment of the inventive film suitable for use in products for packaging or holding while cooking food. is there. The maximum extractable value is 21 CFR 177.
Consistent with current limits for classes of resins intended for use in contact with food products as described and described in 1520, which is incorporated herein by reference in its entirety.

In all of the following examples, the three-layer film was prepared according to the previously mentioned Pahlke US Pat. No. 3,456,0.
It was prepared by the double or trapped bubble method as widely described in No. 44. In particular, all three layers were coextruded simultaneously, cooled, then reheated and biaxially stretched.
Two types of VLDPE were used; Type A was an Attane 4001 consisting of an ethylene and 1-octene copolymer having a density of about 0.912 from the Dow Chemical Company of Midland, Md. Type B VLDPE is also Dow Density about 0.905
Attan 4003 with. The ethylene alpha-olefin plastomers used in these examples are
Density of about 0.88 manufactured by Mitsui Petrochemical Industry Co., Ltd.
Was Tafmer 4085. In a preferred embodiment of the invention, the VLDPE in the outer layer has a density of about 0.
It has a 905 g / cm ^3, ethylene alpha - olefin copolymer plastomer has a density of about 0.88 g / cm ³
Have.

In all of the examples (excluding the prior art WR Grace film), the total thickness of the film sample was about 0.70 mils (0.018 mm) and the first outer layer was about 0.18 mils (0. .0045 mm), the core layer is about 0.
It consisted of 34 mils (0.009 mm) and about 0.18 mil of the second outer layer.

[0059]

EXAMPLE 1 In Example 1, Tafmer A-4085 was blended with VLDPE at various percentages to form first and second outer layers on either side of a core layer of the same VLDPE material. . More specifically, the dry resin outer layer components are first blended by tumble mixing and then fed to a single screw type extruder and die of either 60 mm or 130 mm diameter to feed either VLDPE core resin. Co-extruded onto the side. The smaller die was used only to make samples 1-4 and 7-9, and the larger die was used to make all other samples. The resin is heat plasticized and extruded to a diameter of about 5 inches (13 cm) and a wall thickness of about 10 inches (25
cm) as the primary tube. Extruder barrel and die temperatures are approximately 305 ° to 340 ° F (152 ° to 171 ° C)
Was in the range. Cool primary tube to 68 ° -72
To about 20 ° F (20 ° -21 ° C) and then reheat to about 18
Biaxially stretched at 5 ° to 190 ° F (85 ° to 88 ° C). The machine direction (MD) stretch ratio is also from about 4 to about 1,
And the transverse direction (TD) stretch ratio was about 4-1. The bubble stability was maximized by adjusting the draw point temperature, bubble cooling rate and draw ratio.

The physical and optical properties of the resulting film samples were measured, the heat sealing range was also measured, and the results are summarized in Tables 12 and 13 below.

[0061]

[Table 12]

[0062]

[Table 13]

The heat sealing range was measured to determine and compare the acceptable temperature range for heat sealing plastic films. Sencorp Systems Model 24 from Sencorp Systems Incorporated of Hyannis, Massachusetts
-AS laboratory sealer was used. In this test, width 4
Two inch (TD) samples were cut from the tubular film. The heat sealer was equipped with a time and seal bar pressure regulator. These regulators were set up under the following conditions: Residence time 1.0 seconds Jaw pressure 50 psi (345 kPa)

Referring to the data of Example 1 now summarized in Table I, the Tafmer content in the two outer layers is 0-35.
% Tafmer A-4085-VLDPE Type A
We report the film properties for a biaxially stretched heat-shrinkable film made with a (density 0.912) blend. It should be noted that at the onset, the shrink force,% shrinkage and tear strength all decreased or did not improve as the Tafmer content was increased from 0% to 15%. At 40% Tafmer in sample 7, the primary tube was extruded but it could not be blown and biaxially stretched due to the high Tafmer content. This means that the outer layer is too soft at the stretching temperature,
That is, the film tensile strength was too small. If cooled to a temperature slightly below the stretching temperature,
The extruded tube bursts and, if warmed to just above the draw temperature, the extruded tube produced small holes.

Table J shows that Tafme in the two outer layers
Tafmer A-4085-VL with r content 0-35%
The film properties for a biaxially stretched heat-shrinkable film made with a DPE type B (density 0.905) blend are summarized. VLDPE type B (density 0.90
5) has a lower tensile strength in the form of film than VLDPE type A (density 0.912), so Tafmer4
It was obvious that the outer layer film of 0% -VLDPE type B 60% could not be biaxially stretched. Therefore,
No such experiment was included in Table J.

Comparing the shrinkage percentages, Table I shows that Tafmer blends 10% and 15% (Samples 2 and 3) had approximately the same values, but Tafmer
There is a significant and unexpected increase at level 20% (Sample 4), indicating an even greater improvement at Tafmer level 30% (Sample 5). As an example, the percent shrinkage in the machine and transverse directions for Tafmer 30% Sample 5 is Tafmer 15%.
Significantly about 2.2 and 1.6 times the corresponding values for sample 3, respectively. But Tafmer
The 35% sample 6 still had a much smaller shrinkage value compared to the Tafmer 30% sample 5 but still much larger than the Tafmer 15% sample 3 value.

Comparing the permanent set values, Table I shows Taf
When using mer20% sample 4, Tafmer1
A slightly lower (and improved) value was achieved as compared to 5% Sample 3, indicating that there was a large and unexpected improvement when increasing the Tafmer content to 30%. More specifically, the longitudinal and transverse permanent set values for Tafmer 30% Sample 5 are Tafme
It is about 28% and about 30% smaller than the corresponding values for r15% Sample 3, respectively. Tafmer 35%
The sample 6 value is considerably larger than the permanent strain value of the Tafmer 30% sample 5, but nevertheless Tafmer
It is considerably smaller than the value for the 15% sample 3. Table I also shows that the Tafme 30% sample 5 had Tafme at a sealing range of 30 ° F (17 ° C).
Range 20 ° F (11 ° C) for r15% Sample 3
It is considerably wider than

Comparing the tear strengths, Table I shows Taf
When using mer20% sample 4, Tafmer1
Substantially larger values were obtained compared to the 5% sample 3, ie the MD was about 1.5 times larger and the TD was about 2 times larger. For the Tafmer 30% sample 5, the MD value was about the same as the Tafmer 20% value, while the TD tear strength value was significantly smaller. T
The MD and TD values were both smaller for the afmer 35% sample 6, but remained larger than the tear strength values for the tafmer 15% sample 3.

To summarize the above, Table I of Example 1 includes a blend of VLDPE having a density of 0.912 and about 20 to about 35 wt% ethylene alpha-olefin copolymer plastomer having a density less than about 0.90. It is demonstrated that improvements in percent shrinkage, permanent set, tear strength and sealing temperature range for heat-shrinkable biaxially stretched three-layer films with outer layers are unexpected. I
The table also demonstrates that within this range, optimum properties are achieved when the ethylene alpha-olefin copolymer plastomer comprises about 22 to about 30% of the VLDPE blend. Referring to the data of Example 1 summarized in Table J, the Tafmer content is 0 in the two outer layers.
We report film properties for biaxially stretched heat-shrinkable films made with ˜35% Tafmer A-4085-VLDPE type A (density 0.905) blend. Tear strength and permanent set actually decreased with increasing Tafmer content from 0% to 15%.

The shrinkage percentage is Tafmer 25-
When compared in the range of 35%, Tafmer15 is all
% Was significantly larger than sample 9. For example, Ta
The optimum values achieved with fmer 30% Sample 11 were significantly about 1.6 and about 1.3 times the MD and TD values for 15% Tafmer in the VLDPE blend, respectively. Comparing the permanent set percentages, all values in the Tafmer 25-35% range are T
afmer 15% was significantly and desirably smaller than sample 9. For example, the optimum values achieved with Tafmer 30% Sample 11 were significantly only 58% and 73%, respectively, of the MD and TD values for 15% Tafmer in the VLDPE blend.

Comparing the tear strength values, Table J shows Ta
When fmer content of 25-35% is used, Tafmer
It proves that a considerably higher value was obtained compared to 15%. For example, in Tafmer 35% (Sample 12), the tear values in the MD and TD directions are Tafmer 15%.
About 1.3 times the value for sample 9 and about 2.
It was double. In summary, the Table J data of Example 1 also has an outer layer comprising a blend of VLDPE having a density of 0.905 and about 20 to about 35 wt% ethylene alpha-olefin copolymer plastomer having a density less than about 0.90. We demonstrate that improvements in percent shrinkage, permanent set and tear strength for heat shrinkable biaxially stretched trilayer films are unexpected. Table J also shows
Within this range, the ethylene alpha-olefin plastomer copolymer will comprise from about 22 to about 30 of the VLDPE blend.
Demonstrate that the optimal properties are achieved when constructing%.

Tables I and J show that the films have a shrinkage of at least 30% in both the machine and transverse directions, and most preferably a shrinkage of at least 40% in both the machine and transverse directions. Different embodiments will be demonstrated. Tables I and J also show that the film has a permanent set less than about 7% in at least one of the machine and transverse directions, and most preferably less than about 6% in both the machine and transverse directions. The preferred embodiments of the invention are demonstrated.

Example 2 In these tests, some of the above three-layer film embodiments of the invention were of the type used commercially for shrink-overwrap packaging fresh poultry sections onto plastic trays. The system was tested and their performance compared to a commercially used 5-layer film.

Each system has a length of about 9 inches (23c
m), a width of 6 inches (15 cm) and a polystyrene foam tray having a wall that is approximately 1 1/2 inch (3.8 cm) high and slopes upward and outward to simulate fresh poultry parts. The shaped rubber object was filled. The rubber object was placed in the tray such that it randomly extended about 1 inch (2.5 cm) above the tray wall. These simulated plastic trays containing poultry were conveyed by conveyors at various predetermined linear velocities into molded tubes of packaging film. The tube was formed from a sheet approximately 17 1/2 inches (44.5 cm) wide prior to stretching and was formed by stretching the film transversely around a simulated poultry storage tray. The film was swirled and pulled to the bottom of the tray to heat seal the two longitudinal edges to form a continuous tube with the product in the tray inside. When the transversely stretched film cools, it returns to its original state, forming a tight wrap around the girth of the package.

In Test 2A, a laboratory impulse sealer was powered from the invention tube at 55% power and a residence time of 0.0.
A pouch was made by using it for 80 seconds to form an impulse heat seal at one end. In this particular sealer, the jaws were mechanically actuated by a cam and four heat seal wires were used, two above the film and two below the film. The embodiment of the film used is the sample 5 (Tafmer 4085 3 described above).
0% -blend outer layer of 65% VLDPE with a density of 0.912 and the same VLDPE core layer), Sample 13 (Ta
fmer4085 30% -VLDP with a density of 0.912
E70% blend outer layer and VLDP with 0.905 density
E core layer), and sample 14 (Tafmer 408)
25% -70% VLDPE with 0.912 density-blend outer layer with 5% antifog and same VLDPE core layer).
Hot water was added to each pouch and hung overnight. The next morning, no leaks were noted. This demonstrates that conventional impulse heat sealing can be used to form a leak tight seal between the inner layers of the invention.

In Test 2B, the tubes formed from Samples 5, 13 and 14 above were treated with W. R. Grace and Company-Connecticut Manufactured and SSD-35
Compared to a commercially used 5-layer film labeled 0. The latter film is described in US Pat. No. 4,617,241.
EVA / LLDP as identified in the issue
It seems to have a five-layer structure of E / VLDPE / LLDPE / EVA. Tubes formed from these films were used to form simulated poultry stretched film packages using the laboratory sealer described above with the same impulse sealing and residence time of 0.080 seconds. The seal strength was measured according to the above procedure and the results are shown in Table 1 below.
Summarize in 4.

[0077]

[Table 14]

Table K shows that under equivalent heat impulse sealing conditions, shrink shrink overlap of poultry trayed packages by use of the inventive film as compared to the commercially used five layer film. Demonstrate that a significantly greater seal strength is achieved.

In Test 2C, Samples 5, 13 and 14
A simulated poultry stretched film tray package was made in a commercial sealing system using a tube formed from. This system cuts between the packages, pulls the free end downwards with a vacuum and folds under the package. The package with the tube ends folded is fed onto a hot plate where sealing pressure is applied. Packing conditions are 50 at a sealing temperature of 265 ° F (129 ° C).
Package / min. The three film samples are
All provided packages that were satisfactory from a mechanical suitability and sealing standpoint. Samples 5 and 13 (without the antifog agent described above) were slightly tacky compared to PVC films run under equivalent conditions in this commercial system,
It was noted that it was necessary to adjust the tension. Sample 14 (with the antifogging agent described above) was found to have better mechanical suitability, ie, lower transverse stress, due to the higher slip.

Example 3 In this series of tests, eight different three-layer film embodiments of the invention, including Samples 15-21, were made by the double bubble method under the following conditions: Extrusion Die Temperature-35.
0 ° F (177 ° C), stretching temperature -185 to 190 ° F
(85 ° to 88 ° C.), MD stretch ratio −5: 1, TD stretch ratio −4: 1. Folding diameter 17 1/2 inch (44.5 cm)
Simulated poultry shrink film tray packages were formed using these films with The package was formed using impulse seals at each end. The production rate is about 20 packages / min, and the inventive film is used as sample 22 in W.I. R. A comparison was made with Grace 5-layer commercial film type BDF. This latter film is LLDPE / adhesive / EVOH / adhesive / LLDP
It seems to consist of E. The working conditions for the inventive film embodiment were 260 ° F. (127 ° C.) for the fin seal (ie, the longitudinal seal between the inner layers) and a power setting of 39% / 40% for the impulse seal. Grace-type BDF film has a higher temperature setting, namely:
It required 360 ° F (188 ° C) for fin seals and 83% / 83% power settings for impulse seals. About 20 packages were tested for each type of film.

In Samples 15-21, the VLDPE used in the first and second outer layer blends and the core layer had a density of about 0.912 g / cm ³ Dow type XU.
It was 61520.01. It is the same as type 4001 except that it has a low antioxidant content. The ethylene alpha-olefin plastomer component of the first and second outer layers was Mitsui's Tafmer A-4085.
Samples 15-20 contain Ampacet Corporation's oleamide type slip compound No. 10926 in the first and second outer layers, and Sample 20 contains Ampacet Corporation's behenamide type slip compound No. 100042. . Sample 21 contains ICI type 8112 antifog. Samples 15-21 varied the thickness of the three layers, but the total thickness was either 0.6 mil (0.015 mm) or 0.8 mil (0.
020 mm). Prior art 5-layer film sample 22 had a thickness of 0.6 mils. The properties of film samples 15-22 are summarized in Table 15 below.

[0082]

[Table 15]

These results for Samples 15-22 are summarized in Table 16 below.

[0084]

[Table 16]

From this data, under the same working conditions, poultry shrink film tray packages using the present three layer film have the same sealing efficiency as the more complex five layer film currently in use and the five layer film. I think it is clear that it can be formed with considerably less required power compared to.

Example 4 In this series of tests, an ethylene alpha-olefin copolymer from Exxon Corporation was added to the first and second outer layers of the three layer film in the form of a tube described above and the core layer was 100% VLDPE (Dow type XU61, described above).
520.01). The first and second outer layers were the same in the four samples 23-26 and used the same VLDPE and ethylene alpha-olefin plastomer copolymer Tafmer A-40 used in the core layer.
Consisting of a blend with 85. The exon ethylene alpha-olefin copolymer used in these tests was what the manufacturer describes as "Exact" type 3010C and the three-layer tube was prepared by the same coextrusion double bubble biaxial stretching process used in the previous example. It was made. Exxon Corporation, the manufacturer of the "Exact" type 3010C, is now using this material as a type 30
Identified as No. 27 and provided the following information regarding its physical properties: Melt Index 3.5 (19
2 kg at 0 ° C) and has a density of 0.900 g /
It is cc. It is an ethylene-butene copolymer and has a single melting point of about 92 ° C and a _Mw / _{Mn of} less than about 3. Exxon classifies its Exact material with a density of at least about 0.900 as VLDPE.

The heat sealing range of the heat shrinkable film samples 23 to 26 was determined by measuring the heat seal start temperature and the heat seal burn temperature. Also. Certain physical and optical properties were determined for comparison.
The layer thickness is 0.15 mil (0.0038 mm) for the outer layer and 0.30 mil (0.0076 mm) for the core layer.
And so the total film thickness was about 0.60 mils (0.015 mm) for each sample. The results of this test series are shown in Table 17 below (with Exact added).
Put together.

[0088]

[Table 17]

Table N shows that the addition of Exact 3010C ethylene alpha-olefin to the outer layer of a VLDPE-Tafmer type plastomer blend can significantly reduce the heat seal initiation temperature (Samples 25 and 26 are sampled). (Compare 24). The burn-through temperature is unchanged, so the net effect is to extend the heat sealing range to the desired extent. The heat shrinkage is not substantially changed by adding Exact 3010C, but the tear strength is significantly reduced. In Table N, also sample 23 with an outer layer of EVA 68% -plastomer 29% -slip additive 3% has a very wide sealing range (215 ° -3).
You will notice that it brings about 05 ° F (102 ° -152 ° C). However, it is VLDP in the outer layer
Due to the lack of E, its strength is much smaller than that of samples 24-26 containing VLDPE (compare tear strength). Also, its optical properties are poor (compare haze and gloss) because EVA and ethylene alpha-olefin copolymer plastomers do not mix well.

Example 3 illustrates a bottom section surrounded by upwardly extending side walls, a perishable food product supported on the top surface of the bottom section, and each such food product, the top edge of the side wall and the tray bottom section. Stretched heat-shrinkable film extending over at least a portion of the lower surface and sealed to itself in a stretched relationship to the lower surface so as to form with the tray a hermetically sealed enclosure for food. Demonstrating a hermetically sealed and evacuated food package according to the invention, including a tray having

The improvement in this food package includes at least a first outer layer, a second outer layer and a core layer between the first outer layer and the second outer layer, each ethylene alpha-olefin plast having a density of less than about 0.90. A biaxially oriented multilayer composition comprising a blend of about 20 to about 35 wt% mer copolymer and about 65 to about 80 wt% VLDPE having a density less than about 0.914 is included as a stretched heat shrink film. The core layer comprises an ethylene alpha-olefin copolymer having a melting point above the melting point of either the first or second outer layers.

Although the invention has been described as a three layer film, it should be appreciated that the film may have more than three layers. For example, by circulating the trim and scrap to form a five layer structure, economic benefits may be realized. In particular, a five layer film can be prepared in which the components of the core and the first and second outer layers are as described herein above. The two additional layers intermediate the core layer and each outer layer are formed of recycled trim and scrap of film, at least in part. Thus, these interlayers are usually composed of a blend of VLDPE and ethylene alpha-olefin plastomer copolymer in proportions each corresponding to the proportion of each component in the overall film. However, once the film is illuminated, scrap and trim cannot be circulated in this way.

Further modifications of the invention will be apparent to those skilled in the art and all such modifications are considered to be within the scope of the invention as claimed.

Claims (34)

[Claims] 1. At least a first outer layer, a second outer layer and a core layer between the first outer layer and the second outer layer, wherein the first outer layer and the second outer layer each have a density less than 0.90 g / cm ^3. Comprising a blend of 20-35% by weight of ethylene alpha-olefin plastomer copolymer and 65-80% by weight of VLDPE having a density of less than 0.914 g / cm ³ .
The core layer is a biaxially stretched heat-shrinkable multilayer stretched film containing an ethylene alpha-olefin copolymer having a melting point higher than the melting point of either the first outer layer or the second outer layer. 2. The ethylene alpha-olefin plastomer copolymer comprises 22-30 wt% of the first outer layer and second outer layer blend, and the VLDPE comprises 70-78 wt% of the first outer layer and second outer layer blend. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1. 3. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, wherein the VLDPE has a greater crystallinity than the ethylene alpha-olefin plastomer copolymer. 4. The ethylene alpha-olefin plastomer copolymer has a density of 0.88 g / cm ³ , a melt index of 3.6 g / 10 min. , M _w / M _n 2.
The biaxially stretched heat-shrinkable multilayer stretched film of claim 1 having a Vicat softening point of 35 and a Vicat softening point of 54 ° C. 5. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, wherein the ethylene alpha-olefin plastomer copolymer melts at 55 ° -85 ° C. 6. The VLDPE has a crystal melting point of 117 ° to
The biaxially stretched heat-shrinkable multilayer stretched film of claim 1 having a temperature of 125 ° C. 7. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, wherein the VLDPE has a yield point and the ethylene alpha-olefin plastomer copolymer has no yield point. 8. The biaxially stretched heat-shrinkable multi-layer stretched film according to claim 1, wherein the VLDPE has a broader molecular weight distribution than the ethylene alpha-olefin plastomer. 9. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, wherein the VLDPE has a M _w / M _{n of} at least 5. 10. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, wherein the ethylene alpha-olefin plastomer has a M _w / M _n less than 2.5. 11. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, wherein the ethylene alpha-olefin copolymer plastomer has a Vicat softening point of 50 ° to 65 ° C. 12. The VLDPE has a Vicat softening point of 78 °.
The biaxially stretched heat-shrinkable multilayer stretched film according to claim 1, which has a temperature of -100 ° C. 13. The alpha-olefin plastomer copolymer has a density of 0.88 g / cm ³ and said V
The LDPE has a density of 0.905 g / cm ^3.
The biaxially stretched heat-shrinkable multilayer stretched film. 14. The biaxially stretched heat-shrinkable multi-layer stretched film according to claim 1, wherein the core layer is a polyolefin. 15. The biaxially stretched heat-shrinkable multi-layer stretched film according to claim 1, wherein the core layer is VLDPE. 16. The biaxially stretched heat-shrinkable multi-layer stretched film according to claim 1, wherein the core layer is LLDPE. 17. The biaxially stretched heat-shrinkable multilayer stretched film according to claim 1, wherein the core layer is polypropylene. 18. The biaxially stretched heat-shrinkable multi-layer stretched film according to claim 1, wherein the core layer is a blend of two different ethylene alpha-olefins. 19. The biaxially stretched heat-shrinkable multi-layer stretched film of claim 1, wherein the core layer is a blend of low density ethylene alpha-olefin and high density ethylene alpha-olefin. 20. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, having a shrinkage of at least 30% in both the machine and transverse directions. 21. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, having a shrinkage of at least 40% in both the machine and transverse directions. 22. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1, having a permanent set of less than 7% in at least one of the machine and transverse directions. 23. The biaxially stretched heat-shrinkable multilayer stretched film of claim 1 having a permanent set of less than 6% in both the machine and transverse directions. 24. The biaxially stretched heat-shrinkable multilayer stretched film according to claim 1, which is an irradiated film. 25. The biaxially stretched article of claim 1 including an intermediate layer between the core layer and each of the outer layers, the intermediate layer comprising a blend of the materials of the core layer and the first and second outer layers. Heat-shrinkable multilayer stretched film. 26. A bottom section surrounded by upwardly extending side walls, a perishable food product supported on an upper surface of the bottom section, and each of the food product, a top edge of the side wall and a tray bottom section. Stretched to extend over at least a portion of the lower surface and to itself heat seal in a flattened relationship to the lower surface so as to form with the tray a hermetically sealed enclosure for the food product. In a hermetically sealed and evacuated food package containing a heat-shrinkable film, the biaxially stretched multilayer composition as the stretched heat-shrinkable film comprises at least a first outer layer, a second outer layer and a first outer layer and a second outer layer. includes core layer between the outer layer, said first outer layer and second outer layer each density 0.90 g / cm ³ less than the ethylene alpha --olefin Down plastomeric polymer 20
V less than 35% by weight and density less than 0.914 g / cm ^3.
A food package comprising a blend of 65-80 wt% LDPE, wherein the core layer comprises an ethylene alpha-olefin copolymer having a melting point above the melting point of either the first outer layer or the second outer layer. 27. The food package of claim 26, wherein the stretched heat shrink film is an irradiated film. 28. The stretched heat shrink film comprises an intermediate layer between the core layer and each of the outer layers, the intermediate layer comprising a blend of the core layer and the first and second outer layer materials. Item 26, the food package. 29. The VLDPE has a density of 0.912 g /
a first ethylene alpha-olefin having a cm ³ ;
Density 0.900 g / cm ³ , melt index 3.
27. The food package of claim 26, comprising a blend with a secondary ethylene alpha-olefin that is 5, a butene copolymer having a melting point of 92 [deg.] C and an _Mw / _Mn ratio of less than 3. 30. The first ethylene alpha-olefin comprises 35-40% by weight of the blend and the second ethylene alpha-olefin is 27 of the blend.
30. The food package of claim 29, which comprises-32% by weight. 31. The food package of claim 29, wherein the second ethylene alpha-olefin is Exact type 3010C / 3027. 32. The VLDPE has a density of 0.912 g /
a first ethylene alpha-olefin having a cm ³ ;
Density 0.900 g / cm ³ , melt index 3.
The biaxially stretched heat-shrinkable multilayer stretched film of claim 1 comprising a blend with a secondary ethylene alpha-olefin which is 5, a butene copolymer having a melting point of 92 ° C and an M _w / M _n ratio of less than 3. 33. The first ethylene alpha-olefin comprises 35 to 40 wt% of the blend and the second ethylene alpha-olefin is 27 of the blend.
30. The biaxially stretched heat-shrinkable multi-layer stretched film of claim 29 which comprises .about.32 wt%. 34. The biaxially stretched heat-shrinkable multilayer stretched film of claim 29, wherein the second ethylene alpha-olefin is Exact type 3010C / 3027.